They are disk brakes. The two sets of pads are for service and park brakes:
The Discs attached to the shaft ends of the DC Propel motors are brake discs.
There are two different brakes on each disc. One set is parking and the other is service. Parking brakes are run by hydraulic DOT5 brake fluid. The service brakes are air brakes similar to other air brake applications on heavy duty tracks and busses. The air brakes are controlled by a pedal in the cab. The hydraulic parking brakes are controlled by a switch and it runs a electric over air system. The air is used to depress a diaphram in a cylinder that is attached to a master cylinder, similar to what is in your car. The assembly is called a "Power Cluster" and is simply air over hydraulic.
The parking brakes are smaller than the service brakes and the pads get very little wear since they apply when the crawler is stopped.
The service brakes don't get used very much either, the crawler normally coasts to a stop unless there is an emergency.
Here's a CAD image of the drive train. The two shafts at the top coming forward go into the drive motors. The brakes are on the forward end of the motors.
From maintenance images on the NasaSpaceflight forum:
Last photo of Propel Motors with Brake Discs removed for modifications.